5.4 Multichannel Detectors
Only one spectral channel can be monitored at any instance in time with a
single-channel photodetection element. However, multichannel devices can mea-
sure multiple spectral or spatial channels simultaneously in different time sequences
[ 9 – 11 ]. Two common multichannel devices are based oncomplementary metal-
oxide-semiconductor(CMOS) and charge-coupled device (CCD) technologies.
Both CCD and CMOS devices consist of one-dimensional or two-dimensional
arrays of light-sensitivepixels. These pixels are individual photodetection elements
that convert incoming photons to electrons and collect them in a potential well.
After a certain time period the electrons in the well are counted and converted to an
electrical signal, which corresponds to the optical powerflow onto the device.
5.4.1 CCD Array Technology
A number of architectural variations exist for a CCD. A simple device model for an
individual pixel is shown in Fig.5.7. Each pixel is a metal-oxide-semiconductor
(MOS) capacitor with an optically transparent gate contact, an opaque transfer gate,
Table 5.1 Generic operating parameters of GaP, Si, Ge, and InGaAspinphotodiodes
Parameter Symbol GaP Si Ge InGaAs
Wavelength range (nm) λ 250 – 380 400–1100 800–1650 1100– 1700
Responsivity (A/W) R 0.07–0.1 0.4–0.6 0.4–0.5 0.75–0.95
Dark current (nA) ID 0.01 1 – 10 50 – 500 0.5–2.0
Rise time (ns) τr 500 0.5– 1 0.1–0.5 0.05–0.5
Modulation bandwidth
(GHz)Bm 0.001 0.3–0.7 0.5– 31 – 2Bias voltage (V) VB 55 5– 10 5Table 5.2 Generic operating parameters of Si, Ge, and InGaAs avalanche photodiodes
Parameter Symbol Si Ge InGaAs
Wavelength range (nm) λ 400 – 1100 800 – 1650 1100 – 1700
Avalanche gain M 20 – 400 50 – 200 10 – 40
Dark current (nA) ID 0.1– 150 – 500 10 – 50 @M = 10
Rise time (ns) τr 0.1– 2 0.5–0.8 0.1–0.5
Gain•bandwidth (GHz) M•Bm 100 – 400 2 – 10 20 – 250
Bias voltage (V) VB 150 – 400 20 – 40 20 – 30
NoteSi can be enhanced to extend its operation to the 200–400-nm spectrum, but with a lower
responsivity of about 0.1 A/W for apinphotodiode at 200 nm compared to 0.4–0.6 A/W in the
750 – 950-nm region
5.4 Multichannel Detectors 133